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other detrimental nurture effects or how widespread these effects are on the population.* It may very well be that, despite the dramatic nature of some of the clinical cases, the percent of the population affected in this way may be small. In what follows it will be assumed that the three effects already considered have, in fact, a small impact; for the most part the implications of the replacement effect will be traced.
THE REPLACEMENT EFFECT
The rate at which a population transmits acquired characteristics to subsequent generations will depend in part on the growth rate of the population and its age structure. This is readily seen if we assume that all quality improvements take place among the lower age groups and not the higher ones. For example, nurture and schooling improvements are, for the most part, likely to enter the system during early ages. To the extent that entrants into the work force are of higher quality (i.e., higher education and acquired skills, etc.) than those that leave through retirement or death, the average quality of the labor force improves more rapidly if the rate of population growth is higher (other things equal) rather than lower.
"There is some evidence on the effects of starvation that can lead to interesting calculations. For example, Keys (24) determines experimentally the reduction in physical capacities as a consequence of "semistarvation," e.g., a shift from 3,000-plus calories to about 1,600 calories leads in 12 weeks to a decrease in strenuous physical work capacity to 52 percent of the nonstarved group, and, at the end of 24 weeks, to a decrease equal to only 28 percent of the nonstarved group. Of course the initial level based on the diet of well-off American students is unusually high. Perhaps the work capacity is not much lower for a standard below the U.N. Food and Agriculture Organization (FAO) norm but above Colin Clark's (25) calculated norm for west Asia. If so, then we might readily visualize an agricultural family producing grain that would yield 9,600 calories per family per day. Calculating children as two thirds of adults, this would lead a three-child family to have approximately 2,400 calorics per adult-equivalent whereas something close to a six-child family would reduce the intake to the semistarvation level of about 1,600 calories. At this level we might apply Keys' results with qualifications. In the Carnegie experiment mentioned by Keys, men's physical capacities were reduced trivially when on a 2,000 calorie low-weight-maintenance diet. See Brown (26) for actual nutritional reference diets, pp. 36, 142-143. See also Clark (25, pp. 123-129). If we raise Clark's figures by some 25 percent for the greater weight and height of Americans to tally with Keys' data, we obtain a norm of about 2,300 calories.
It seems probable that the greatest effects of "semistarvation" are the psychological ones. In Keys' experiments, apathy was a major consequence of semistarvation. It was highly correlated with such psychological elements as lack of ambition, decrease in sell-discipline, decrease in mental alertness, and a decrease in concentration. On a self-rating scale the increase in apathy and the related psychological characteristics was roughly calibrated at about 1.75 on a range between no apathy (0) and extremely more apathy (5). This was approximately half of the amount of the sense of tiredness reported. Of course, it is very difficult to determine the economic significance of these results, but they are suggestive of the possible relationship between poor diet and the type of resultant characteristics that inhibit economic change.